It is vital to develop an anti-loosing nut. In order to achieve an anti-loosing effect, we investigate an interference of thread by plastic forming. However, when the deformation of thread is too large, it is impossible to fasten the nut due to the excessive torque. The method of plastic forming is an axial compression of nuts with a slit in circumferential direction Thus, in this study, the effects of the geometrical properties of the slit nut are investigated by experiment and using the finite element method. As a result, we have proposed the optimum shape of.slit nuts with the anti-loosing effect.